This invention relates to a clad product formed of a base and a cladding of dissimilar materials, and the method of manufacture, and more particularly, to a clad product of predetermined closely-toleranced shape and dimensions manufactured by dimensionally pre-profiling at least the base or cladding material to compensate for dimensional distortion occurring during solid-phase bonding of the base and cladding together.
Clad products are commonly used where certain desired properties in a finished product are critical, but which cannot be obtained by that product being formed of just the base material. Therefore, there have been manufactured composite finished products formed, for example, of two dissimilar metals. The cladding technique of forming this type of composite product is to bond together the two dissimilar metals by suitable pressure and temperature so that at the interface each metal normally diffuses sufficiently into the other to produce a bond. The composite of a product having a substantially thin dissimilar metal cladding on the outer surface of a metal base is extremely suitable for obtaining different electrical or thermal conductivities, tensile or torsional strengths, corrosion resistance, wearability, and other desired physical and chemical product properties.
Wire-type clad products which comprise a circular metal rod core and an outer thin dissimilar metal sheath bonded around the rod core are of significant commercial and industrial importance. Examples of the wire-type clad product are telephone, electronic, and electrical coaxial cable for carrying a double signal. To obtain the electrical and other desirable properties for such cable, not only is the metal composition of the cable critical, but dimensional requirements, such as the concentricity of the clad product and thickness of the outer thin metal cladding sheath, must be held within very close tolerances.
All cladding products, e.g. wire and cable, must, of course, be manufactured in the most cost-effective way while ensuring the necessary composition and dimensional requirements. Two key factors in reducing cost are to provide a simple manufacturing method and to eliminate unnecessary material in the product itself or wastage occurring during manufacture. Product material waste becomes a substantial cost factor when it is necessary or desirable to make the outer cladding sheath of a precious or other expensive metal or material.
There have been developed generally three principal methods of forming clad products of two dissimilar metals, frequently referred to as (1) casting-and-rolling, (2) clad-casting, and (3) solid-cladding. In manufacturing a rod clad product by casting-and-rolling, the outer sheath is molten cast onto the base material in the form of a billet, the billet and outer cladding being subsequently rolled to form the composite product. The clad product produced by this process is, however, unsatisfactory by not having a perfect concentricity. An elliptical shape in the cross section of the product frequently occurs with non-uniform thickness in the outer clad sheath around the core due to pressure variation during rolling. This problem is further compounded by variation in the thicknesses of the outer clad sheath along the longitudinal direction of the rod clad product as a result of the tapering effect caused by the rolling process. That is, a thickness of the outer clad sheath is greatest at a position along that portion of the clad product which is formed in the middle of the billet with a thinner thickness being formed along those portions of the product formed at the ends of the billet. Finally, there is the problem of the rolling process squeezing off the outer cladding material at the billet ends, commonly referred to as self-cropping, thus resulting in a wastage in the cladding material during manufacture.
A rod clad product manufactured by clad-casting also proves unsatisfactory. Clad-casting involves feeding a cold-drawn or pre-formed circular core through a die into a receptacle where molten cladding material is then cast or coated around the core, the coated core than passing through an exit die with the composite rod clad product being taken up by a coiler for storage and subsequent wire drawing. With this method of manufacture, the outer clad sheath again has a non-uniform thickness and the concentricity of the end rod clad product is adversely affected. Several principal factors cause these unsatisfactory results. First, there is unevenness in the wear of the dies, at both the inlet and outlet, during casting of the molten cladding material. Secondly, there is a "guitar-string" effect caused by the rod core being fed under tension through the dies of the molten cladding material and into the coiler, causing a phenomenon of "chatter" or "shimmy" occurring in the casting of the molten cladding material around the core. This results in a cladding unevenness.
In an effort to overcome the problems encountered in the casting-and-rolling and clad-casting methods of manufacturing, solid-cladding has been developed.
One type of solid-cladding process is taught in U.S. Pat. No. 3,714,701 issued to Dion et al. This solid-cladding process forms a circular rod clad product by using a pair of continuous lengths of thin flat metal cladding strips, bending them into semi-circular shapes around a circular metal rod core, and then solid-phase bonding the cladding strips to the rod core. This solid-cladding process, however, again proves unsatisfactory to provide a circular rod clad product with a closely-toleranced concentricity and a uniform thickness in the outer cladding sheath. Product material waste also occurs in this process due to the non-uniformity of the sheath thickness.
The shortcomings encountered by using a solid-cladding process are caused by the key step of solid-bonding the two dissimilar materials. Solid-phase bonding requires that a sufficient pressure be exerted by rollers on the cladding strips for bonding the cladding strips against the circular core and for sealing the edge contacts of the cladding strips.
Because uniform circumferential pressure is not applied to all surfaces of a rod having a circular transverse shape during conventional rolling operations, concentrations of pressures result in different material flows in at least the cladding or core. These material flows can produce variations in thickness of the cladding layer. Thus, necessary pressure exerted by the rollers thins portions of the cladding strips so that a closely-toleranced concentricity of the end product and a uniform thickness for the cladding cannot be obtained. Furthermore, the varying circumferential pressures applied by the rollers can deform the core, thereby causing a non-concentric end product.
Frequently, a specification requires a minimum thickness for the outer cladding sheath, as well as uniformity. The thinning effect caused by solid-phase bonding would then require formation of an outer cladding sheath which has portions thicker than necessary in order to meet the overall minimum thickness standard. Excessive thickness in portions of the outer sheath, of course, creates wastage in the cladding material and a higher cost for the manufacture of the composite clad product.
In summary, the conventional clad products and their methods of manufacture have not proven completely satisfactory. Formation of a clad product having predetermined closely-toleranced shape and dimensions made in a cost-effective way has yet to be achieved. The severe limitations in the conventional methods of forming clad products are particularly prevalent in forming a circular rod clad product of closely-toleranced concentricity with uniform thickness for the outer cladding sheath. Difficulties attributed to the conventional methods of manufacture have been compounded recently by the significant increase in cost of the cladding and core materials and in the imposition of close dimensional tolerances and surface finish specifications for clad products now being developed for different commercial and industrial uses.